In a typical cellular communications system, wireless user equipment units (UEs), for example, mobile phones, communicate via a radio access network to one or more core networks. A radio access network covers a geographical area which is divided into cells, with each cell area being served by a radio base station. Several base stations are connected, typically via land lines, to a control node known as a radio network controller (RNC). Such a control node supervises and coordinates various activities of the several radio base stations which are connected to it. The radio network controllers are typically connected to one or more core networks. One example of a radio access network is the Universal Mobile Telecommunications (UMTS) Terrestrial Radio Access Network (UTRAN). The UMTS is a third generation (3G) system and UTRAN is essentially a radio access network providing wideband code division multiple access (WCDMA) to user equipment units. Fourth generation systems are evolving towards a broadband and mobile system. The 3rd Generation Partnership Project has proposed a Long Term Evolution (LTE) solution, namely, an Evolved Universal Mobile Telecommunication System Territorial Radio Access Network, (E-UTRAN), for a mobile access network.
In many radio access networks the radio base station is a concentrated node with most of its components being located at a concentrated site. However, a radio base station can also be configured with a more distributed architecture. For example, a distributed radio base station can take the form of one or more radio equipment (RE) portions that are linked to a radio equipment control (REC) portion over an internal interface. One example of an internal interface of a radio base station which links a radio equipment portion of the radio base station to a radio equipment control portion of the base station is the Common Public Radio Interface (CPRI). The Common Public Radio Interface (CPRI) is described in Common Public Radio Interface (CPRI) Interface Specification Version 4.1 (18 Feb. 2009) and also Version 4.2 (2010) and Version 5 (2011).
The Common Public Radio Interface (CPRI) is an industry cooperation aimed at defining a publicly available specification for the key internal interface of radio base stations between radio equipment control (REC) and radio equipment (RE), thereby allowing base station manufacturers to share a common protocol and more easily adapt platforms from one customer to another. In essence, a radio base station is decomposed into two separate blocks, known as REC and RE. The REC provides access to a UMTS network, for example, via the Iub interface, whereas the RE serves as the air interface to user equipment, known as Uu in a UMTS network. The REC generally comprises the radio functions of the digital baseband domain, whereas the RE contains analogue radio frequency functions. The functional split between the REC and RE is done in such a way that a generic interface, CPRI, based on In-Phase and Quadrature (IQ) data can be defined. Several IQ data flows can be sent over one physical link with each data flow reflecting the data of one antenna for one carrier, the so-called antenna carrier “AxC.” Several AxC's having the same sampling rate may be aggregated into an “AxC Group.” IQ data of different antennas along with control data are multiplexed onto a transmission line. The CPRI has a basic frame structure for carrying a control word and an IQ data block.
The functional split between the REC and RE allows the RE to be positioned close to an associated antenna. This reduces the distance which the associated signals have to travel before they are received by the RE, thereby negating the need for tower-mounted amplifiers and antenna system controllers. The link between the RE and REC is generally optical, allowing the link length to be much greater when compared with wired coaxial systems. Therefore, the distance between the RE and RRC can be around 10 Km, thereby increasing the flexibility of deployment of RE's within the network when utilising CPRI. One REC may be linked to two or more RE's or one RE may be linked to multiple REC's in a chain topology with each REC being configured to forward data to other REC's in the chain.
Many of the functions which an REC has to perform, which include channel coding/encoding, spreading/despeading, frame and time slot generation, for example, may be realised by a proprietary digital signal processing device. Two examples of such DSP devices which support the CPRI are the Freescale B4860 and the Freescale MSC 8157 Broadband Wireless, Access Six Core DSP which is described in Freescale Semiconductor Data Sheet MSC8157E, November/2011. This Digital Signal Processor includes (inter alia) a CPRI unit which includes a “framer” module which handles the transmission and reception of all IQ data, and a DMA (direct memory access) module associated with the framer which transfers the receive and transmit IQ data to and from antenna carrier buffers in a memory.
Usually, the sampling rates of all AxC's in a CPRI unit are the same. A single CPRI unit will normally support only one AxC sample rate. If different sampling rates need to be supported, however, additional circuitry is required. One known way of supporting two sampling rates in the MSC8157 device employs three CPRI units. FIG. 1 shows an example of this known solution. In the receive direction, a first framer 101 receives RF data (from an RE 102 for example) and its associated DMA module 103 transfers data with a first sampling rate to system memory 104. A second framer 105 receives the RF data from the first framer 101 via auxiliary interface modules 106, 107 and outputs it directly via Serialiser/Deserialiser links 108, 109 to a third framer 110. A DMA module 111 associated with the third framer 110 transfers data with a second sampling rate to system memory 112. In the transmit direction, the DMA module 103 associated with the first framer 101 fetches data with the first sampling rate from system memory 104 and transfers it to the first framer 101 for outputting towards the RE 102. The third framer's DMA module 111 fetches data with the second sampling rate from system memory 112 and the third framer 110 transfers the data to the second framer 105 via the serialiser/deserialiser 108, 109 interface. Then, the second framer 105 transfers the data with the second sampling rate via the auxiliary interfaces 107, 106 to the first framer 101 which, in turn, outputs the data towards the RE 102. Each CPRI unit contains receive and transmit configuration tables that determine which AxC's in a frame to transfer to or fetch from system memory and their location in the frame. Further, each CPRI unit contains receive mask registers (not shown) with the masking of bits of the auxiliary interface 106, 107. For example, if a masking bit is “set” then data coming from framer2 105 is sent to the RE 102. Conversely, if the masking bit is “cleared” then the data read by framer1 101 from system memory 104 is sent to the RE 102. This solution has the disadvantage of requiring an extra CPRI unit and interconnections.